2. Description of the Related Art
Ozone has long been recognized as a useful chemical commodity valued particularly for its outstanding oxidative activity. Because of this activity, it finds wide application in disinfection processes. In fact, it kills bacteria more rapidly than chlorine, it decomposes organic molecules, and removes coloration in aqueous systems. Ozonation removes cyanides, phenols, iron, manganese, and detergents. It controls slime formation in aqueous systems, yet maintains a high oxygen content in the system. Unlike chlorination, which may leave undesirable chlorinated organic residues in organic containing systems, ozonation leaves fewer potentially harmful residues. Ozone has also been shown to be useful in both gas and aqueous phase oxidation reactions which may be carried out by advanced oxidation processes (AOPs) in which the formation of OHxe2x80xa2 radicals is enhanced by exposure to ultraviolet light. Certain AOPs may even involve a catalyst surface, such as a porous titanium dioxide photocatalyst, that further enhances the oxidation reaction. There is even evidence that ozone will destroy viruses. Consequently, it is used for sterilization in the brewing industry and for odor control in sewage treatment and manufacturing. Ozone may also be employed as a raw material in the manufacture of certain organic compounds, e.g., oleic acid and peroxyacetic acid.
Thus, ozone has widespread application in many diverse activities, and its use would undoubtedly expand if its cost of production could be reduced. For many reasons, ozone is generally manufactured on the site where it is used. However, the cost of generating equipment, and poor energy efficiency of production has deterred its use in many applications and in many locations.
On a commercial basis, ozone is currently produced by the silent electric discharge process, otherwise known as corona discharge, wherein air or oxygen is passed through an intense, high frequency alternating current electric field. The corona discharge process forms ozone through the following reaction:
3/2O2=O3;xcex94Hxc2x0298 =34.1 kcal
Yields in the corona discharge process generally are in the vicinity of 2% ozone, i.e., the exit gas may be about 2% O3 by weight. Such O3 concentrations, while quite poor in an absolute sense, are still sufficiently high to furnish usable quantities of O3 for the indicated commercial purposes. Another disadvantage of the corona process is the production of harmful NOx otherwise known as nitrogen oxides. Other than the aforementioned electric discharge process, there is no other commercially exploited process for producing large quantities of O3.
However, O3 may also be produced through an electrolytic process by impressing an electric current (normally D.C.) across electrodes immersed in an electrolyte, i.e., electrically conducting fluid. The electrolyte includes water, which, in the process dissociates into its respective elemental species, O2 and H2. Under the proper conditions, the oxygen is also evolved as the O3 species. The evolution of O3 may be represented as:
3H2O=O3+3H2;xcex94Hxc2x0298=207.5 kcal
It will be noted that the xcex94Hxc2x0 in the electrolytic process is many times greater than that for the electric discharge process. Thus, the electrolytic process appears to be at about a six-fold disadvantage.
More specifically, to compete on an energy cost basis with the electric discharge method, an electrolytic process must yield at least a six-fold increase in ozone. Heretofore, the necessary high yields have not been realized in any foreseeable practical electrolytic system.
The evolution of O3 by electrolysis of various electrolytes has been known for well over 100 years. High yields up to 35% current efficiency have been noted in the literature. Current efficiency is a measure of ozone production relative lo oxygen production for given inputs of electrical current, i.e., 35% current efficiency means that under the conditions stated, the O2/O3 gases evolved at the anode are comprised of 35% O3 by weight. However, such yields could only be achieved utilizing very low electrolyte temperatures, e.g., in the range from about xe2x88x9230xc2x0 C. to about xe2x88x9265xc2x0 C. Maintaining the necessary low temperatures, obviously requires costly refrigeration equipment as well as the attendant additional energy cost of operation.
Ozone, O3, is present in large quantities in the upper atmosphere in the earth to protect the earth from the suns harmful ultraviolet rays. In addition, ozone has been used in various chemical processes and is known to be a strong oxidant, having an oxidation potential of 2.07 volts. This potential makes it the fourth strongest oxidizing chemical known.
Because ozone has such a strong oxidation potential, it has a very short half-life. For example, ozone which has been solubilized in waste water may decompose in a matter of 20 minutes. Ozone can decompose into secondary oxidants such as highly reactive hydroxyl (OHxe2x80xa2) and peroxyl (HO2xe2x80xa2) radicals. These radicals are among the most reactive oxidizing species known. They undergo fast, non-selective, free radical reactions with dissolved compounds. Hydroxyl radicals have an oxidation potential of 2.8 volts (V), which is higher than most chemical oxidizing species including O3. Most of the OHxe2x80xa2 radicals are produced in chain reactions where OHxe2x80xa2 itself or HO2xe2x80xa2 act as initiators.
Hydroxyl radicals act on organic contaminants either by hydrogen abstraction or by hydrogen addition to a double bond, the resulting radicals disproportionate or combine with each other forming many types of intermediates which react further to produce peroxides, aldehydes and hydrogen peroxide.
Electrochemical cells in which a chemical reaction is forced by added electrical energy are called electrolytic cells. Central to the operation of any cell is the occurrence of oxidation and reduction reactions which produce or consume electrons. These reactions take place at electrode/solution interfaces, where the electrodes must be good electronic conductors. In operation, a cell is connected to an external load or to an external voltage source, and electric charge is transferred by electrons between the anode and the cathode through the external circuit. To complete the electric circuit through the cell, an additional mechanism must exist for internal charge transfer. This is provided by one or more electrolytes, which support charge transfer by ionic conduction. Electrolytes must be poor electronic conductors to prevent internal short circuiting of the cell.
The simplest electrochemical cell consists of at least two electrodes and one or more electrolytes. The electrode at which the electron producing oxidation reaction occurs is the anode. The electrode at which an electron consuming reduction reaction occurs is called the cathode. The direction of the electron flow in the external circuit is always from anode to cathode.
Recent ozone research has been focused primarily on methods of using ozone, as discussed above, or methods of increasing the efficiency of ozone generation. For example, research in the electrochemical production of ozone has resulted in improved catalysts, membrane and electrode assemblies, flowfields and bipolar plates and the like. These efforts have been instrumental in making the electrochemical production of ozone a reliable and economical technology that is ready to be taken out of the laboratory and placed into commercial applications.
However, because ozone has a very short life in the gaseous form, and an even shorter life when dissolved in water, it is preferably generated in close proximity to where the ozone will be consumed. Traditionally, ozone is generated at a rate that is substantially equal to the rate of consumption since conventional generation systems do not lend themselves to ozone storage. Ozone may be stored as a compressed gas, but when generated using corona systems the pressure of the output gas stream is essentially at atmospheric pressure. Therefore, additional hardware for compression of the gas is required, which in itself reduces the ozone concentration through thermal and mechanical degradation. Ozone produced by the corona process may also be dissolved in liquids such as water but this process generally requires additional equipment for introducing the ozone gas into the liquid, and at atmospheric pressure and ambient temperature only a small amount of ozone may be dissolved in water.
Because so many of the present applications have the need for relatively small amounts of ozone, it is generally not cost effective to use conventional ozone generation systems such as corona discharge. Furthermore, since many applications require either ozone gas to be delivered under pressure or ozone dissolved in water as for disinfection, sterilization, treatment of contaminants, etc., the additional support equipment required to compress and/or dissolve the ozone into the water stream further increases system costs. Also, in some applications, it is necessary to maximize the amount of dissolved ozone in pure water by engaging ozone gas in chilled water under pressure. This mode of operation can minimize the amount of pure water required to dissolve a large amount of ozone. Such highly concentrated aqueous solutions of ozone can be added to a stream of process water to maintain a desired concentration of ozone in the process water stream.
Therefore, there is a need for an ozone generator system that operates efficiently on standard AC or DC electricity and water to deliver a reliable stream of ozone gas that is generated under pressure for direct use in a given application. Still other applications would benefit from a stream of highly concentrated ozone that is already dissolved in water where it may be used directly or diluted into a process stream so that a target ozone concentration may be achieved. It would be desirable if the ozone generator system was self-contained, self-controlled and required very little maintenance. It would be further desirable if the system had a minimum number of moving or wearing components and could be operated with a minimal control system.
The present invention provides an ozone generation and delivery system comprising an electrochemical cell having an anode and a cathode, a cathode reservoir in communication with the cathode, and a cooling member disposed in thermal communication with the cathode reservoir. The cathode may form a portion of the cathode reservoir. Where the cathode reservoir is a cathode phase separator chamber, it may include a gas outlet port disposed in a top portion of the cathode phase separator chamber, with the cathode disposed in a bottom portion of the cathode phase separator chamber. Furthermore, the cathode phase separator chamber should provide fluid communication with the cathode allowing the free exchange of water and gas bubbles between the cathode and the cathode phase separator chamber. The ozone generation and delivery system may further comprise an anode reservoir in fluid communication with the anode, a water source in fluid communication with the cathode reservoir, a water source in fluid communication through a backflow prevention device to the anode reservoir, and/or a pressure control device in the gas outlet port. Optionally, the anode reservoir may comprise a liquid reservoir, a gas outlet port located at the top of the reservoir and a porous hydrophobic membrane disposed over the gas outlet port, wherein the porous hydrophobic membrane of the anode reservoir allows gas to be separated from water while the water is contained under pressure. The system may also comprise a recycle line providing fluid communication from the cathode phase separator to the anode reservoir, perhaps with a backflow prevention device in the recycle line. Preferably, the recycle line has a sufficiently small diameter to prevent dissolved ozone from diffusing from the anode reservoir to the cathode phase separator. It is also preferable that the cathode and anode are separated by a proton exchange membrane. The system may comprise a pressure regulating member disposed in the gas outlet. Additionally, the anode reservoir may further comprises a water outlet near the base of the anode reservoir.
The present invention also provides an ozone generation and delivery system comprising a plurality of electrochemical cells, each cell having an anode and a cathode, and further comprising an anode reservoir in communication with the anodes, the anode reservoir preferably comprising a gas outlet port and a porous hydrophobic membrane disposed over the gas outlet port. The plurality of electrochemical cells are preferably placed in a filter press arrangement, but may also be placed with the anodes forming a portion of the anode reservoir.
Additionally, the present invention provides an electrochemical method of generating and delivering ozone, comprising the steps of (a) electrolyzing water in one or more electrolytic cells to generate ozone in water at the anode and a cathodic product in water at the cathode; (b) circulating water between the cathode and the cathode reservoir; and (c) cooling water in the cathode reservoir. The hydrogen gas may be separated from the cathode water using a porous hydrophobic membrane disposed in the cathode phase separator. The method may further comprise back diffusing water from the cathode through a proton exchange membrane to the anode, wherein the proton exchange membrane is disposed between the cathode and anode. Further, the method may comprise delivering ozone gas from the anode under pressure and/or delivering the anode water from the anode reservoir under pressure. The method may also include destroying surplus ozone and hydrogen.
Another aspect of the present invention provides an electrochemical cell, comprising a compressible electrode, a rigid electrode, and a proton exchange membrane compressed between the compressible electrode and the rigid electrode. Preferably, the compressible electrode and rigid electrodes contain a fluid, wherein the fluid in the compressible electrode is under greater pressure than the fluid in the rigid electrode. The electrochemical cell may further comprise a rigid support member behind the compressible electrode reducing the compression on the compressible electrode.
Finally, the present invention provides a method of operating an electrochemical cell, comprising: (a) providing a reactant to an electrode at a flowrate substantially equal to consumption of the reactant, and (b) periodically flushing the electrode with the reactant at a flowrate substantially greater that consumption of the reactant to remove compounds accumulated on the electrode. Optionally, the method may further comprise (c) monitoring the flowrate of reactant into the electrode.